Glossary

Atoms are
the building blocks of the universe. About 92% of the atoms in space are
Hydrogen , about 7.8% are Helium, and the remaining 0.2% are the other elements
like Carbon, Nitrogen, Oxygen,.... Hydrogen is an atom that is composed of one
electron bound to one proton. If a region is hot enough, the electrons can be
stripped from the proton creating hydrogen ions. If a region is cold and dense
enough, pairs of hydrogen atoms will combine into hydrogen molecules. Most of
the hydrogen in this room is in the form of molecules.

False colorsis
a technique scientist use to help them see details in images of objects. The
colors the scientist picks for his or her image may have nothing to do with the
color of the object. Sometimes the chosen colors represent the temperature of
parts of the object. Sometimes, the density. And so on. Thus, the viewer of a
false color image must always be told by the creator of the image what the
colors mean.

Hydrogen makes up 92% of the atoms in space.
Hydrogen can be in one of three forms: either molecular, atomic, or ionized.
Many of the characteristics of an object in interstellar space depends upon the
form that hydrogen is in.

Infrared light
is another part of the electromagnetic spectrum and lies beyond what humans can
see as red light. Infrared light can be produced by objects with temperatures
from about zero to 2000 degrees Fahrenheit. In fact, our bodies produce infrared
light that can be observed using special devices. Some nocturnal animals have
eyes that can see the infrared light produced by their prey or enemies.

Ionsare
created whenever a region becomes very hot (e.g., over a few thousand degrees).
Ions are atoms whose electrons have been stripped from them because of the high
temperature.

Interstellar dust
make up only 1% of the interstellar medium but, because space is so big,
sometimes there is enough dust to obscure the light from stars behind the dust.

Interstellar gasoccupies
the space between stars. It is a very thin gas with, on the average, one atom in
a volume the size of your thumb. The gas is mostly hydrogen (92%), some helium
(8%), and very little of other kinds of atoms. The interstellar medium is is
made up of interstellar gas and dust.

Interstellar matter
or the interstellar medium is made up of mostly interstellar gas (99%) and dust
(1%). The gas comes in patches that astronomers call interstellar clouds. Clouds
can be made mostly out of ions, atoms, or molecules. Stars form out of the
densest clouds which astronomers call molecular clouds. Most of what we do in
radio astronomy is the study of the interstellar medium.

Molecular clouds
are patches of the interstellar medium that are so dense that molecules can
form. These clouds are very cold, typically just a 10 to 100 degrees above
absolute zero (about minus 225 degrees Celsius or minus 375 degrees Fahrenheit).
In molecular clouds, the gas gets dense enough that stars can form out of the
gas.

Molecules
form when two or more atoms combine together. Most of the molecules in space can
only exist in molecular clouds where the interstellar medium is cold and dense.

Nebulae is
a term astronomers use to describe all kinds of irregularly-shaped, hazy patches
of interstellar matter. Some glow by a physical process similar to what happens
in fluorescent light bulbs. Some are cold clouds of interstellar gas that are
dense enough that the dust particles in the gas obscure the light from stars
beyond the cloud -- these regions look like, but are not, holes in space.
Sometimes the edges of dark nebula glow because they reflect starlight
(reflection nebulae).

Radio Waves are almost identical to the light
waves we see with our eyes. The only difference is the wavelength of the wave.
Radio waves have a wavelength of between about a 0.1 inch to many yards. Optical
light, on the other hand, has a wavelength of about 0.00002 inches. Both optical
and radio waves travel at the speed of light (186,000 miles/sec) and can travel
through a vacumn. A common misconception is that radio waves is a form of sound
waves. Sound waves cannot travel through a vacumn and, in air, travel at about
1000 feet/sec. The misconception comes about since humans take sounds waves and
convert them to and from radio waves in radio broadcasting. What travels between
your local radio station and your radio receiver is radio waves. Your radio
receiver then converts the radio waves back to sound waves.

Star Formation
is the process by which stars are created. Astronomers believe stars form out of
the densest regions of the interstellar medium, the molecular clouds.

Supernova
is a term for one of the most violent events we know of. Supernovae occur when a
large star is at the end of its life and explodes. The energy of such an
explosion is greater than the combined energy put out by billions of normal
stars. Most stars, and in particular our Sun, will not explode like this since
they are not large enough, not massive enough, to produce an explosion. Instead,
stars like the Sun will have a less violent, but still a violent enough, death.